Optical circuits formed of silicon waveguides have been actively researched and developed since they have an advantage that the circuit size can be significantly smaller than those using conventional materials such as silica. Controlling the polarization of passing light, in particular, polarization splitting and polarization rotation, is one of important functions of an optical circuit. A polarization rotator is a circuit configured to output light of polarization perpendicular to the polarization of polarized light inputted thereto. As optical circuits formed on a planar substrate, generally used is an optical circuit configured to convert TE-polarized light, having its electric-field component horizontal to the substrate, into TM-polarized light, having its electric-field component vertical to the substrate, or an optical circuit configured to convert TM-polarized light into TE-polarized light.
There have been proposed several methods of performing polarization rotation with a silicon waveguide. As a first conventional technique example, a circuit has been proposed which is configured to adiabatically rotate polarization by using a structure of silicon nitride Si3N4 added on a silicon waveguide. Details of this polarization rotator are disclosed in NPL 1.
Moreover, as a second conventional technique example, a polarization rotator has been proposed which includes a polarization converter and a mode converter with silicon waveguides whose over claddings are made of silicon nitride. In this conventional technique example, the polarization converter includes a tapered waveguide whose width gradually increases, while the mode converter includes an asymmetric directional coupler including two waveguides differing in width. This polarization rotator is disclosed in detail in NPL 2.
FIGS. 43A and 43B are views showing the configuration of the polarization rotator in the above second conventional technique example. FIG. 43A is a top view of a surface where the circuit is formed. The polarization rotator shown in FIG. 43A includes: a polarization converter 1003 formed of a tapered waveguide whose width increases from w1 to w2; and a mode converter 1014 formed of an asymmetric directional coupler including two waveguides 1005, 1006 differing in width. The tapered waveguide 1003 and the waveguide 1005 are smoothly connected by a second tapered waveguide 1004. Optical signals of given polarizations and modes are inputted into and outputted from the polarization rotator shown in FIG. 43A through input-output waveguides 1001, 1007, 1008. The input-output waveguide 1001 and the tapered waveguide 1003 are smoothly connected by a third tapered waveguide 1002. In the figure, the waveguides and the boundaries of their regions are individually illustrated with clear dotted lines (partitions perpendicular to the direction of travel of light). It is, however, to be noted that the partitions are illustrated based on the functions for convenience and the constituent material has no discontinuity or actual boundary.
In an example of the actual structure, the length of the tapered waveguide 1003 is 50 μm, and its width is w1=0.69 μm to w2=0.83 μm. The waveguide 1005 and the waveguide 1006, forming the asymmetric directional coupler, have a width of 0.9 μm and a width of 0.405 μm, respectively, and both have a length of 17 μm. The gap between the two waveguides 1004, 1005 is 0.25 μm. The width of the input-output waveguide 1001 is 0.5 μm, and the length of each of the second tapered waveguide 1004 and the third tapered waveguide 1002 is 15 μm.
Also, in a different structure example focusing on reducing the entire circuit length, the length of the tapered waveguide 1003 is 44 μm, and its width is w1=0.69 μm to w2=0.83 μm as in the above structure. The widths of the waveguide 1005 and the waveguide 1006, forming the asymmetric directional coupler, are 0.9 μm and 0.405 μm, respectively. The gap between the waveguides 1005, 1006 is 0.15 μm so that the lengths of the two waveguides 1005, 1006 can both be 7 μm and the length of asymmetric directional coupler can therefore be reduced. The width of the input-output waveguide 1001 is 0.54 μm. The length of each of the second tapered waveguide 1004 and the third tapered waveguide 1002 is 15 μm. In the case of this structure, the entire length of the polarization rotator is 71 μm.
FIG. 43B shows a cross-sectional structure taken along line XLIIIB-XLIIIB in the top view of the polarization rotator shown in FIG. 43A. An under cladding 1010 made of silica is provided on a silicon substrate 1011. A core 1003 of the tapered waveguide made of silicon is formed on the under cladding 1010. Further, an over cladding 1009 of silicon nitride Si3N4 is formed in such a way as to cover the core 1003. The core thickness of the waveguide 1003 is 0.22 μm, the thickness of the over cladding 1009 is 1.5 μm, and the thickness of the under cladding 010 is 2 μm.
TM-polarized basic mode light 1012 inputted into the circuit from the input-output waveguide 1001 is converted into TE-polarized 1st order mode light as it propagates through the tapered waveguide 1003, or the polarization converter. Further, at the asymmetric directional coupler 1014, or the mode converter, the TE-polarized 1st order mode light propagating through the waveguide 1005 is coupled to a TE-polarized basic mode at the waveguide 1006, and TE-polarized basic mode light 1013 is outputted into the input-output waveguide 1008. The circuit shown in FIGS. 43A and 43B is bidirectional. Thus, in a case where the TE-polarized basic mode light 1013 is inputted in the reverse direction from the input-output waveguide 1008, it passes through paths that are the reverse of the paths mentioned above, and the TM-polarized basic mode light 1012 is outputted into the input-output waveguide 1001.
A polarization rotator needs to have an asymmetric waveguide in order to cause polarization conversion of an optical wave propagating therethrough. A structurally symmetric waveguide has no coupling between a TE mode and a TM mode and cannot therefore cause polarization conversion. In Conventional Technique Example 1, the silicon nitride Si3N4 structure formed on top acts to break symmetry. In Conventional Technique Example 2, the under cladding and the over cladding differ from each other in refractive index and thereby provide asymmetry.